The present invention relates to a control system for the supercharger of an internal combustion engine and, more particularly to a system for detecting, directly or indirectly, the state of combustion of the air-fuel mixture in the internal combustion engine and controlling the supercharge pressure supplied by the supercharger in accordance with the state of combustion.
For improving the fuel efficiency or output characteristics of an engine, it is effective to supercharge the engine by use of a supercharger such as a turbo charger having a turbine driven by the energy of the exhaust gas and a compressor directly coupled thereto. The supercharger, which is generally used for improving the output at the time of high speed drive of the engine, may be used for increasing the torque at the time of a low or middle speed drive. The rotational speed of the turbocharger driven by the exhaust gas increases with increasing of the engine speed, and therefore if an effective supercharge is to be attained at low or middle speed of the engine, the supercharge pressure may increase excessively and often damage the engine at high speeds. Generally, when the supercharge pressure is increased, the state of combustion of the air-fuel mixture in the engine changes in such a way, that, for example, the average pressure in the cylinder increases. It has been proposed, therefore, to detect the average pressure in the cylinder and when this pressure increases excessively, part of the exhaust gas supplied to the turbocharger is bypassed to thereby decrease the supercharge pressure. In the case of the internal combustion engine of spark ignition type, the combusion timing of the air-fuel mixture changes and a knocking occurs more easily when the supercharge pressure increases excessively. In order to obviate this inconvenience, it has also been proposed that when a knocking is detected by a knock sensor, part of the exhaust gas is bypassed to thereby reduce the supercharge pressure. The method of detecting the state of combustion of the engine directly or indirectly and controlling the supercharge pressure according to the result of detection is more desirable than the method of detecting the engine rotational speed or rotational speed of the turbocharger and controlling the supercharge pressure in accordance with the result of detection. This is because some variation in performance of the turbocharger is unavoidable and it is difficult to control the change in supercharge pressure caused by such a variation in performance in the latter method. In the former method, the output of, for example, a knock sensor for detecting the condition of a knocking, is fed back to an actuator for driving a valve for bypassing the exhaust gas, and thus the valve opening is changed thereby to control the supercharge pressure.
In the prior art, the pressure in the intake manifold of the engine, downstream of the throttle valve, namely, the supercharge pressure and the atmospheric pressure are introduced, in combination, as the control pressure for the actuator for driving the exhaust gas bypass valve. In the absence of a knocking, the proportion of the atmospheric pressure introduced is increased so that the resultant control pressure is decreased so as to close the valve to thereby attain as high a supercharge pressure as possible. Upon occurrence of a knocking, the introduction of atmospheric pressure is stopped and only the supercharge pressure is introduced so that the exhaust gas bypass valve is opened. A sufficient margin must be provided, in design of the system, taking the possible variation in performance of the turbo charger into consideration, to prevent a knocking when the control pressure reaches maximum, namely, when the control pressure is given by only the supercharge pressure. For the purpose of achieving a high output and a lower fuel rate of the engine, however, it is desirable to maintain the supercharge pressure at a condition where the engine operates with a light knock or the engine slightly knocks. In these methods of control, the range of adjustment is limited by the characteristics of the pressure in the intake manifold and it is difficult to control the engine to a light knock state over the range from low to high speeds thereof.
Further, at the time of adjusting the control pressure, the intake manifold communicates with the atmosphere, which results in that part of the mixture gas or air to be introduced into the engine being discharged into the atmosphere. If such a method is used for the electronics fuel injection system in which the amount of intake air is measured at the inlet of the intake manifold to control the amount of fuel injection, an accurate control of air-fuel ratio is impossible. The same is true in the case where the supercharge pressure is controlled in accordance with the average pressure in the cylinder. In other words, although it is desirable to run the engine with as high an average pressure in the cylinder as possible in order to achieve a higher output and a lower fuel rate, the above-mentioned conventional methods require, in design, sufficient margin to maintain the average pressure in the cylinder below the safety limit even when the maximum control pressure is applied to the valve actuator, thus making impossible to satisfy the above-mentioned demand or to achieve accurate control of the air-fuel ratio for the reasons mentioned above.